Switching circuit for rotary magnetic heads



April 1967 B. E. GUISINGER 3,316,358

SWITCHING CIRCUIT FOR ROTARY MAGNETIC HEADS Filed May 17, 1963 2 Sheets-Sheet 1 sup/2W6 5205/4 H4 /2 H2 z I I 60/05 /8 TA PE 20 H3 aurpur comm-20.4 C/ECU/TO SYA/CHPQ/V/ZEE 5/122577 5 GU/S/NGEE IN VEN TOR.

BY flaw/024% Arm/WE) OUTPUT B. E. GUISINGER April 25, 1967 SWITCHING CIRCUIT FOR ROTARY MAGNETIC HEADS 2 Sheets-Sheet 2 Filed May 17, 1963 w Ev .1 @Ewmmi Iii FIIIIIIIIIIIIIIIIIII IIIIIIII uh \DUW \\.U QQQKQQIU 5466 577 5 GU/S/A/GER INVENTOR.

ATTOP/I/Ey United States Patent 3,316,358 @WTTCHING CIRCUIT FOR ROTARY MAGNETIU HEADS Barrett E. Guisinger, Redwood City, Calif., assignor to Ampex Corporation, Redwood City, Calif., a corporation of California Filed May 17, 1963, Ser. No. 281,146 12 Claims. (Cl. 179-1lltl.2)

This invention relates generally to switching apparatus and particularly to a high speed electronic switching circuit.

The prior art is replete with innumerable switching mechanisms including a great number of electronic switching circuits. Despite the availability of these prior art switches, the need continually arises for improved switches to enhance the state of the various arts in which switches are employed. For example, in transverse scan television recorders in which a plurality of magnetic heads are disposed on the periphery of a scanning drum, a very fast switch or commutating device is provided for sequentially coupling each of the heads to an output circuit.

Various switch designs have been proposed for use in this video recording application and several have proved to be satisfactory. However, in many instances the satisfactory performance was short-lived since, as other portions of the recording were improved, formerly unobjectionable noise and transients introduced by the switches become objectionable. Consequently, a constant effort is exerted to develop new switch designs to keep pace with progress in related arts. The Journal of the Society of Motion Picture and Television Engineers, vol. 66, April 1957, in an article beginning on page 184, discusses in detail the problems encountered in designing switches for recorders employing a magnetic head scanning drum. Typically, the most significant problems lay in the inability of existing switches to operate suificiently rapidly, coupled with their tendency to introduce large voltage transients resulting from the relatively long recovery time required subsequent to switching.

In view of the deficiencies of prior art switch designs, it is an object of the present invention to provide a high speed electronic switching circuit that avoids the introduction of large voltage transients.

Briefly, the invention is directed to an extremely rapid switching circuit that is characterized by the absence of large switching transients during operation. This characteristic is achieved by the use of a selectively actuable clamping device connected to a point in an impedance path, coupling a switch input terminal to a switch output terminal, such that when the clamping device is actuated, variations in an input voltage applied to the input terminal do not appear at the output terminal and conversely when the clamping device is not actuated, input voltage variations do appear at the output terminal.

In a preferred embodiment of the invention, the clamping device includes a pair of oppositely poled transistors, each having their emitters and collectors connected between the p-oint in the impedance path and a clamp reference voltage. The bases of the transistors are respectively connected to opposite output terminals of a binary device, such as a multivibrator, so that for one state of the binary device both transistors are saturated, and for the other state of the binary device, both transistors are cut off. When the transistors are saturated, the point in the impedance path between the switch input and output terminals is clamped thereby preventing input voltage variations from appearing at the output terminal. When the transistors are cut off, input voltage variations do appear at the output terminal.

3,316,353 Patented Apr. 25, 1967 The invention will be described in greater detail with reference to the accompanying drawings, in which:

FIGURE 1 is ascheniatic diagram of a portion of a television or wideband recorder in which an electronic switching circuit constructed in accordance with the present invention can be utilized;

FIGURE 2 is a schematic diagram illustrating the manner in which the magnetic heads of the wideband recorder of FIGURE 1 are connected to switching means; and

FIGURE 3 is a circuit diagram illustrating the manner in which switching circuitry constructed in accordance with the present invention can be connected to the magnetic heads of FIGURE 1.

Attention is now called to FIGURE 1 that illustrates a portion of a wideband recorder, such as described in US. Patent No. 2,866,012, that includes a scanning drum 10 carrying magnetic heads H1, H2, H3, and H4, uniformly positioned around the drum It) at intervals. Rotatable along with the wheel ltl is a slip ring 12. A 'brush 14 is conductively engaged with the slip ring 12 and adapted to couple electrical signals appearing on the slip ring 12 to an output conductor 16.

The drum 10 is spaced from a guide 18 such that a magnetic tape 20 can be passed through the gap therebetween. That is, the tape 20 is adapted to be moved in a direction parallel to the axis about which the drum 10 rotates.

The heads H1, H2, H3 and H4 successively scan the tape 20 transversely. Magnetic fields oriented on the tape 29 so as to represent desired information initiate an electrical current in the winding (not shown) on each head as the head moves across the tape. Although serial connection of the head windings is theoretically feasible, in practice, because of noise effects and a slight overlapping of the intervals during which each head traverses the tape, it is not feasible to connect all of the head windings in series. Rather, it is imperative that some type of switching mechanism be associated with the heads so as to assure that adjacent heads do not simultaneously contribute signals to the output conductor 16.

An initial and extremely simple switching arrangement is to segment the slip ring such that each segment can be connected to a different head winding. Such an arrangement is unsatisfactory because of the switching speeds required as a result of the desired bandwidth of the system, and because of the size limitations of the slip ring.

Moreover, due to the switching speed requirements, the utilization of electronic switching techniques as opposed to electromechanical techniques, suggests itself. Two electronic switching circuits could be utilized with the heads of FIGURE 1 in the manner shown in FIGURE 2. More particularly, the windings 011 heads H1 and H3 could be connected to a common slip ring 22 while the windings on heads H2 and H4 could be similarly connected to a common slip ring 24. Brushes '26 and 28 respectively associated with slip rings 22 and 24 are connected through a control circuit 30 to an output conductor 32. The control circuit Gil includes switches S1 and S2 which respectively connect brushes 26 and 28 to the output conductor 32. The switches S1 and S2 are under the control of a synchronizer 34 in the control circuit which functions to close the switches S1 and 82 alternately and in synchronism with the rotation of the heads thereby alternately coupling the signals applied to the slip rings 22 and 24 to the output conductor 32. Since opposite heads are connected to the same slip ring, the switching arrangement shown in FIGURE 2 avoids the possibility that overlapping signals will be applied to the output conductor 32.

Attention is now called to FIGURE 3 which illustrates a preferred form of switching circuitry that can be utilized I? in the arrangement of FIGURE 2. Again, the windings on heads H1 and H3 are connected through slip ring 22 to a brush 2 6. The brush .26 is connected through a preamplifier (not shown) to the base of a transistor Q1 and to ground through a resistor 36. The collector of transistor Q1 is connected to a source of negative potential while the emitter thereof is connected through resistor 38 to a source of positive potential. Additionally, the emitter of transistor Q1 is connected through serially connected resistors 40 and 42 to the base of a transistor Q2. The emitter of transistor Q2 is connected through a resistor 44 to a source of positive potential while the collector thereof is connected to a source of negative potential.

The junction between resistors 40 and 42 is connected to the collectors of transistors Q3 and Q4 respectively. The emitters of transistors Q3 and Q4 are connected to ground. It is pointed out that transistors Q3 and Q4 are of opposite polarity types.

The base of transistor Q3 is connected through a parallel circuit including resistor 46 and capacitor 48 to a first output terminal 50' of a multivibrator 52. Similarly, the base of transistor Q4 is connected through a parallel circuit consisting of resistor 54 and capacitor 56 to a second output terminal 58 of the multivibrator 52. The resistances 40 and 42 connected in series between the emitter of transistor Q1 and the base of transistor Q2 together with the transistors Q3 and Q4 and associated circuitry comprise the switch S1 referred to in FIGURE 2.

The switch S2 connecting the brush 28 to the base of transistor Q2 through a preamplifier (not shown) is identical to the switch S1. That is, the brush 28 is connected to the base of transistor Q1 and through resistor 36' to ground. The emitter of transistor Q1 is connected through resistor 38 to a source of positive potential and through serially connected resistors 40 and 4 2 to the base of transistor Q2. The junction between resistors 40 and 42 is connected to the collectors of transistors Q3 and Q4 while the emitters of these transistors are connected to ground. The base of transistor Q3 is connected through resistor 46 and capacitor 48 to the first terminal 50 of the multivibrator 52. It is to be noted that transistors Q3 and Q3 are of opposite polarity types. Similarly, the transistor Q4 is poled oppositely to that of transistor Q4. The base of transistor Q4 is connected through resistor 54 and capacitor 56 to the second output terminal 58 of multivibrator 52.

In the operation of the embodiment of FIGURE 3, the transistors Q3 and Q4 of switch S1 are both saturated for one state of the multivibrator 52 and are both out off for the opposite state of the multivibrator 52. More particularly, for a first state of the multivibrator 52, a positive potential will be applied to the output terminal 50 and a negative potential will be applied to the output terminal 58 so as to cut off both transistors Q3 and Q4. When transistors Q3 and Q4 are cut off, any potential change at the emitter of transistor Q1 will appear at the base of transistor Q2. On the other hand, for an opposite state of the multivibrator that is with a negative potential applied to the output terminal 59 and a positive potential applied to the output terminal 58, the junction between resistors 40 and 42 will be clamped at approximately ground potential and voltage variations at the emitter of transistor Q1 will not appear at the base of transistor Q2.

Since the transistors in switch S2 are arranged oppositely to those in switch S1, when the transistors of switch S1 are saturated, the transistors of switch S2 will be cut off, and conversely when the transistors of switch S2 are saturated, the transistors of switch S1 are cut off. Consequently, voltage variations appearing at the emitters of transistors Q1 and Q1 will alternately appear at the base of transistor Q2.

Currents induced in the windings on the heads H1 and H3 are coupled to the base of transistor Q1 so as to modulate the emitter-collector current passed therethrough. That is, with no current induced in the head windings, an emitter-collector current in the transistor Q1 will exist to establish the emitter thereof at a certain voltage level. If the potential on the slip ring 22 is increased, it will reduce the magnitude of the emittercollector current in transistor Q1. On the other hand, if the potential on slip ring 22 is reduced, the emitter collector current through transistor Q1 will be increased to thereby reduce the transistor Q1 emitter voltage.

By mounting the heads H1, H2, H3 and H4 on the periphery of a scanning drum as shown in FIGURE 1, signals will be successively induced in each head winding as the head traverses the tape 20. That is, initially the current induced in the winding on head H1 will modulate the emitter voltage of transistor Q1 and this modulated voltage will be coupled through resistors 40 and 42 to the base of transistor Q2. Simultaneously, the transistors of switch S2 will be saturated thereby clamping the junction between resistors 40 and 42 so as to prevent any currents induced in the windings of heads H2 and H4 from afiecting the base of transistor Q2. After head H1 traverses the tape 20, the multivibrator 52 switches state so as to saturate the transistors of switch S1 and cut off the transistors of switch S2. Consequently, the current induced in the windings on head H2 will modulate the voltage on the emitter of transistor Q1 which will appear at the base of transistor Q2. From a voltage standpoint it can be said that when the transistors Q3 and Q4 of switch S1 are saturated, the voltage at the junction between resistors 40 and 42 is clamped thereby preventing voltage variations at the emitter of transistor Q1 from appearing at the base of transistor Q2. From a current standpoint the transistors Q3 and Q4 can be considered as a circuit for diverting (when saturated) the current out of the series path including resistors 40 and 42.

The voltage appearing at the base of transistor Q2 will proportionately modulate the transistor Q2 emitter voltage to thereby permit an output signal to be derived across the emitter and collector of transistor Q2.

From the foregoing it should be appreciated that an improved switching apparatus has been disclosed herein, which finds particular utility in magnetic tape recorder systems for rapidly and successively connecting each magnetic head carried on a scanning drum to an output circuit. The switching can be accomplished very rapidly (about 40 nanoseconds) while maintaining any voltage transients at a minimum.

Although the switching circuitry herein has been dis closed in conjunction with a magnetic tape recording system, it is pointed out that the circuitry finds utility in other diverse applications in which it is essential to provide an extremely fast acting electrical switch.

What is claimed is:

1. An electronic switch including an input terminal; an output terminal, first and second impedance means directly serially connected between said input and output terminals; and means for selectively clamping the junction between said first and second impedance means whereby signals applied to said input terminal are prevented from appearing at said output terminal, said means for selectively clamping including at least one transistor having its collector and emitter connected between said junction and a clamp reference voltage means.

2. An electronic switch including an input terminal; an output terminal, first and second impedance means directly serially connected between said input and output terminals; and means for selectively clamping the junction between said first and second impedance means whereby signals applied to said input terminal are prevented from appearing at said output terminal, said means for selectively clamping including first and second transistors; means for supplying a clamp reference voltage; and means for connecting the emitters and collectors respectively of said first and second transistors in parallel between said junction and the reference voltage means for preventing said junction from swinging above or below the reference voltage by more than the emitter-collector voltage drop across a saturated transistor; and means for alternately saturating and cutting off said first and second transistors.

3. The switch of claim 2 wherein said first and second transistors are of opposite polarity types and have their emitters connected to said reference voltage means and their collectors connected to said junction.

4. The switch of claim 2 wherein said means for alternately saturating and cutting off said transistors includes a multivibrator having first and second output terminals; and means respectively connecting said first and second multivibrator output terminals to the bases of said first and second transistors.

5. In combination with a plurality of magnetic heads fixedly supported on the periphery of a scanning drum adjacent to a magnetic tape, and an output circuit; switching apparatus for successively coupling each of said heads to said output circuit comprising: a like plurality of input circuits, each of said heads connected to an input circuit; a plurality of switches, each of which is uniquely connected to a different one of said input circuits; each of said switches comprising first and second impedance means serially connected between one of said input circuits and said output circuit; and means for selectively clamping the junction between said first and second impedance means whereby signals derived from said input circuits are prevented from appearing at said output circuit.

6. The combination of claim 5 wherein said means for selectively clamping includes first and second transistors; means for providing a clamp reference voltage; and means for connecting said first and second transistors between said junction and the reference voltage means for preventing said junction from swinging above or below the reference voltage by more than the emittercollector voltage drop across a saturated transistor; and means for alternately saturating and cutting off said first and second transistors.

7. The combination of claim 6 wherein each of said input circuits comprises a transistor; means forward biasing the emitter-collector path of said input circuit transistor; and means connecting at least one of said heads to the base of said input circuit transistor.

8. The combination of claim 6 wherein said output circuit comprises a transistor; and means connecting said second impedance means remote from said junction to the base of said output circuit transistor.

9. An electronic switch for switching between first and second input terminals and a common output terminal comprising a first pair of impedances: serially connected between said first input terminal and said output terminal, said first pair of impedances having a first junction therebetween, a second pair of impedances serially connected between said second input terminal and said output terminal, said second pair of impedances having a second junction therebetween, and means for alternately clamping said first and second junctions whereby signals applied to said first and second input terminals are alternately prevented from appearing at said output terminal.

10. An electronic switch according to claim 9, further defined by said means for clamping comprising first and second transistors each having base, emitter, and collector, a clamp reference voltage source, said first transistor having its emitter and collector connected between said first junction and said reference source, said second transistor having its emitter and collector connected between said second junction and said reference source, and means connected to said bases of said first and second transistors for alternately saturating same.

11. An electronic switch according to claim 10, further defined by the means for saturating comprising a multivibrator having first and second output terminals, and means respectively connecting said first and second multivibrator output terminals to the bases: of said first and second transistors.

12. An electronic switch according to claim 11, further defined by third and fourth transistors each having base, emitter and collector, said third and fourth transistors being of opposite polarity type to said first and second transistors, said emitter and collector of said third transistor respectively connected to the emitter and collector of said first transistor, said emitter and collector of said fourth transistor respectively connected to the emitter and collector of said second transistor, and means respectively connecting said second and first multivibrator output terminals to the bases of said third and fourth transistors.

References Cited by the Examiner UNITED STATES PATENTS 2/1963 Rywak 307-885 1/1966 Bounsall 179-1002 BERNARD KONICK, A. I. NEUSTADT,

Assistant Examiners, 

5. IN COMBINATION WITH A PLURALITY OF MAGNETIC HEADS FIXEDLY SUPPORTED ON THE PERIPHERY OF A SCANNING DRUM ADJACENT TO A MAGNETIC TAPE, AND AN OUTPUT CIRCUIT; SWITCHING APPARATUS FOR SUCCESSIVELY COUPLING EACH OF SAID HEADS TO SAID OUTPUT CIRCUIT COMPRISING: A LIKE PLURALITY OF INPUT CIRCUITS, EACH OF SAID HEADS CONNECTED TO AN INPUT CIRCUIT; A PLURALITY OF SWITCHES, EACH OF WHICH IS UNIQUELY CONNECTED TO A DIFFERENT ONE OF SAID INPUT CIRCUITS; EACH OF SAID SWITCHES COMPRISING FIRST AND SECOND IMPEDANCE MEANS SERIALLY CONNECTED BETWEEN ONE OF SAID INPUT CIRCUITS AND SAID OUTPUT CIRCUIT; AND MEANS FOR SELECTIVELY CLAMPING THE JUNCTION BETWEEN SAID FIRST AND SECOND IMPEDANCE MEANS WHEREBY SIGNALS DERIVED FROM SAID INPUT CIRCUITS ARE PREVENTED FROM APPEARING AT SAID OUTPUT CIRCUIT. 